The communications spacecraft must be maintained at, or at least near, particular orbital stations in order to provide the service for which they are designed. The spacecraft tend to deviate from their assigned stations due to gravitational effects of the sun and the moon, and also for other reasons. Stationkeeping maneuvers are used to keep the spacecraft on-station. If the spacecraft cannot be kept on station, it may lose utility and have to be replaced. Replacement spacecraft are very costly both to construct and to launch.
Stationkeeping maneuvers may be required in any direction, but one well-known type of maneuver which may be required on an approximately weekly basis is the North-South stationkeeping maneuver. Typically, the spacecraft will require operation of ΔV thrusters to provide thrust in a northerly or southerly direction. Such thrust may be provided by ΔV thrusters mounted on the nominal south face of the spacecraft. FIG. 1 illustrates the south, anti-earth, west, and east faces of a spacecraft S. In FIG. 1, the ΔV thrusters on the south face are designated 13, 14, 15, and 16. In order to make maximum use of the available stationkeeping propellant, the mass specific impulse of the south-face ΔV thrusters is maximized. This may be accomplished, for example, by the use of an electrical arc to heat the reacting propellant. Such ΔV thrusters are known as arcjets. During a north-south stationkeeping maneuver, the south-face arcjets 13, 14, 15, and 16 are typically operated continuously, with a 100% duty cycle.
Operation of the ΔV thruster(s) unavoidably results in torques on the spacecraft. These torques result when the spacecraft center of mass is not aligned with the center of pressure of the ΔV thrusters, and also due to impingement of the thruster plumes on the solar panels of the spacecraft.
In addition to stationkeeping, spacecraft require attitude control. Attitude refers to the orientation of the spacecraft body in space. Attitude control is required for reasons associated with the communication payload, and also for more mundane reasons such as the need to keep the south-face arcjets 13, 14, 15, and 16 pointed in a southerly direction. Several schemes are known for maintaining a given attitude, including the use of magnetic torquers and attitude control thrusters. Magnetic torquers may not have sufficient torque for some applications, and depend upon the earth's magnetic field, which may vary from place to place. Attitude control thrusters can be used to directly control the attitude, but the very low magnitudes of thrust which are ordinarily required are such that even small attitude control thrusters must be used at low duty cycles. When used at low duty cycles, attitude control thrusters are inefficient in their use of propellant. Inefficient use of propellant is to be avoided, as the useful life of the spacecraft depends upon how long stationkeeping and attitude control propellant lasts.
In FIG. 1, 5-pound-force (Lbf) thrusters designated 17, 18, 19, 20, 21, and 22 are illustrated as being mounted on the anti-earth face of spacecraft S. Also, 0.2 Lbf thrusters designated 1, 2, 3, and 4 are mounted at the corners of the south face, similar thrusters 5, 6, 7, and 8 are mounted at the corners of the east face, and likewise thrusters designated 9, 10, 11, and 12 are mounted at the corners of the west face.
Improved spacecraft maneuver attitude control is desired.